A Modular Dynamics Tester (MDT) is an instrument used to acquire aliquots of reservoir fluid for analyses and transportation. The reservoir fluid is drawn into the MDT through a probe in-contact with the bore-hole wall by reducing the pressure within the MDT tubular, which contains bore-hole fluid, from the pressure of the formation. The pressure reduction is generated by a positive displacement pump operated by hydraulic fluid. This is a positive displacement pump that has two pistons within a cylinder wherein one piston contacts a hydraulic fluid and the other piston contacts the flow-line fluid.
The position of the piston is not determined as a function of displacement. When the fluid within the tubular is free of drilling fluid, as determined by the interpretation of independent measurements on the flow-line, the reservoir fluid is directed into the sample bottle. The position of the piston within the sample bottle and thus the intake of fluid are not currently determined.
Prior art methods and apparatus determine the quantity of hydraulic or lubricating fluid contained within a compensator using sound speed measurements. This measurement is required because the hydraulic fluid is continually ejected in the bore-hole through rotary shaft-seals. This approach is shown schematically in FIG. 1.
Referring to FIG. 1, a cross-section through the bellows C and pressure vessel containing either hydraulic or lubricating fluid A is used in a reservoir fluid sampling-while-drilling instrument. An acoustic transducer T is mounted flush with the inner surface of the pressure vessel. The sound is reflected, by the acoustic impedance mis-match, at the metallic surface R of the bellows and travels a distance 2,/before arrival at the transducer T that is now acting as a receiver. The surface R is on a bellows that is parallel with the surface of T and moves within the cylinder A. The bellows serves to both separate the MUD from the hydraulic or lubricating fluid and transmit the pressure within the bore-hole to the hydraulic and lubricating fluids.
In an alternative prior art configuration, illustrated in FIG. 2, an apparatus is described to determine the position of the pump piston surface utilizing measurements of the time of flight of a pulse of sound combined with knowledge of the speed of sound. In FIG. 2, a cross-section through a displacement pump used to move reservoir and bore-hole fluid B with a hydraulic fluid A is provided. An acoustic transducer T is mounted flush with the screen surface S. The sound is reflected due to the acoustic impedance mismatch, at the metallic surface R and travels a distance 2/ before arrival at the transducer T that is now acting as a receiver. The surface R is on a piston that is parallel with the surface of T and moves within the cylinder C.
In another prior art alternative embodiment, an apparatus is described to determine the position of the piston surface within a sample bottle, shown schematically in FIG. 3, from measurement of the time of flight of a pulse of sound combined with knowledge of the speed of sound.
A cross-section through a sample bottle used to transport reservoir fluid E containing hydraulic fluid A is illustrated in FIG. 3. This particular bottle is used in the Modular Dynamics Tester. An acoustic transducer T is mounted flush with the surface S. The sound is reflected, by the acoustic impedance mismatch, at the metallic surface R and travels a distance 2/ before arrival at the transducer T that is now acting as a receiver. The surface R is on a piston that is parallel with the surface of T and moves within the cylinder F. Although not shown, the sample bottle is also fitted with measurements of temperature and pressure.
Conventional systems and methods utilize a time-of-flight determination of the distance separating a transducer and reflector within a fluid for which the sound speed is known. The choice of this method, wherein the transducer used both emits and detects the acoustic wave, is mounted directly into one end of the pressure vessel. This approach requires a method to interconnect the transducer with the processing electronics that might require either wire or wireless communication. For the case of the pump shown in FIG. 3, the cylinder is external to the main apparatus and it is located within a bay that is exposed to bore-hole fluid. This arrangement significantly reduces the time required to exchange one pump for another. Any connection between the transducer and the tool housing would require wires and electrical feedthroughs that both offer additional potential failure modes for operation of the apparatus.